feat(apple/library): launch a picked title (step 4 client side)

Tapping a game in the (flagged) library now starts a session that asks the host to
launch it — the picked GameEntry id rides the connect down to the host, which resolves
it against its own library (27e5865).

- PunktfunkConnection.init gains `launchID` and calls the new punktfunk_connect_ex4
  (wrapping it in withOptionalCString; nil = host default).
- Threaded SessionModel.connect(launchID:) → ContentView.connect(_:launchID:) →
  a `launchTitle(host, id)` helper that dismisses the browser and connects.
- LibraryView gains `onLaunch`; cards become buttons that fire it. Wired on every
  platform (ContentView sheet on macOS/iOS, HomeView destination on tvOS) via a new
  `onLaunchTitle` closure on HomeView. Settings footer updated (launch is live now).

Can't compile Swift on the Linux box; CI (apple.yml) verifies. The host side of this
chain is live-validated on the dev box: a client `--launch custom:<id>` made the host
resolve the id and spawn gamescope running the title (see 27e5865).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

clients/apple/Sources/PunktfunkClient/HomeView.swift

@@ -24,6 +24,8 @@ struct HomeView: View {
     let connectDiscovered: (DiscoveredHost) -> Void
     /// Pairing succeeded (tvOS PairSheet route) — pin + connect (ContentView guards staleness).
     let onPaired: (StoredHost, Data) -> Void
+    /// Picked a title in the (experimental) library — start a session that launches it.
+    let onLaunchTitle: (StoredHost, String) -> Void
     /// Experimental game-library browser (gated) — the host-card "Browse Library…" action.
     @AppStorage(DefaultsKey.libraryEnabled) private var libraryEnabled = false
 
@@ -85,7 +87,7 @@ struct HomeView: View {
                 SpeedTestSheet(host: host)
             }
             .navigationDestination(item: $libraryTarget) { host in
-                LibraryView(store: store, host: host)
+                LibraryView(store: store, host: host, onLaunch: { onLaunchTitle(host, $0) })
             }
             #endif
             #if !os(tvOS)